1. Field of the Invention
The present invention relates to an encrypted communication system with which a created plain document is encrypted before being transmitted to a viewer and the encrypted document is decrypted and the plain document is received on a viewer side, as well as to an encrypted communication method of the encryption system, an encrypting device used in the encrypted communication system, and a decrypting device used in the encrypted communication system.
2. Description of the Related Art
In general, plain documents created in organizations such as companies include various kinds of information, e.g. contents that can be opened to the public (outside the company), contents that are not disclosed to the outside the company, such as employee and customer information, contents such as management information that is disclosed only to those who are in the executive positions in that company organization. Particularly, information that is important in that company and considered as highly confidential is considered as necessary to be encrypted from the security view point, and it is common to encrypt the entire plain document (the entire part of a file) to keep the security.
However, a single plain document may include the content which can be read freely and the content which is preferred not to be read, in accordance with levels of the authorized powers of members (executives, ordinary employees, external workers, etc.) even those are members of a same organization. In that case, even if the document is encrypted for the security reason, it can be read by all the viewers when the encrypted file is decrypted.
Further, when an exclusive document is created for each viewer, it is necessary to correct all of each document for making a correction in a part that is common to the plain documents of all the viewers. This work is complicated and may cause errors in correction. If the documents are distributed with such miscorrection, wrong information is dispersed. Therefore, it takes a lot of cost as well as time and effort to clear the problem.
As encrypted communication system techniques which restrict viewing of a specific range within an article in accordance with levels of the authorized powers of the viewers, there are techniques depicted in Patent Documents 1-3, for example.
The encrypted communication system depicted in Japanese Unexamined Patent Publication 2002-259634 (paragraphs [0051]-[0059]: Patent Document 1) includes an information distributing device that belongs to a slip issuer and information distributing devices that belong to each of a plurality of viewers. With the technique of Patent Document 1, first, the information distributing device of the slip issuer creates viewable range identifiers for identifying viewable ranges for permitting the viewing of the slip for each viewer, and decrypting information which shows a list of pairs of identifiers and common keys for encrypting the viewable ranges. This decrypting information is encrypted by using a public key of each viewer and it is transmitted to each viewer. The information distributing devices of the viewers decrypt the received decrypting information by using own private keys, respectively, to obtain the common key for decrypting the viewable range that is permitted to itself.
Then, the information distributing device of the slip issuer encrypts the respective part in the slip designated by the viewable range identifiers by using the corresponding common keys, and transmits those to the respective viewers. The information distributing devices of the viewers respectively decrypt the received viewable range (permitted to view) of the slip by using the common key from the decrypting information that is obtained earlier. In this manner, the creator of the slip sets different decrypting information for each user for allowing each user to view only the specific part of the slip that is defined by the respective viewable range identifiers.
With the encrypted communication system depicted in Japanese Unexamined Patent Publication H05-244150 (FIG. 3, FIG. 4, and the like) (Patent Documents 2) and Japanese Unexamined Patent Publication H09-233067 (FIG. 3, FIG. 4, and the like) (Patent Documents 3), viewable range identifiers that vary depending on the levels of the authorized powers of the viewers are written in advance in the slip that is used for communication. The creator of the slip creates an encrypted document by encrypting the part that is sandwiched by the identifiers by using an encrypting key that corresponds to the level of the authorized power of a prescribed viewer, and transmits the encrypted document to that viewer. Then, the viewer decrypts the encrypted part by using the encrypting key that corresponds to the level of the authorized power of the viewer oneself. In this manner, the creator of the plain document restricts the viewable range depending on the viewers.
However, with those communication systems described above, the identifiers are inserted into the plain documents to be transmitted to the viewers for designating the range that is encrypted by using the identifiers, which makes it complicated for the creators to create the encrypted documents. Further, when the identifiers are inserted into the plain documents, the creator and the viewers who decrypt the encrypted documents are to read the plain documents having the identifiers included therein, which may make it difficult for them to understand the contents of the plain documents.